The present invention relates to a prism for deflecting a flux of light, and a viewing optical system using the prism, such as binoculars, camera finders, or the like.
Conventionally, a prism for the viewing optical system is, in order to prevent a ghosting light, provided with an aperture member on a light emerging surface or light incident surface thereof. Further to the above, unused surfaces (surfaces which are not used for transmitting and/or reflecting light) of the prism are formed to be a rough surface having minute concave and convex portions in order to reduce reflectance thereof, and further black ink is applied thereon.
For example, in FIG. 4, a prism 100 is provided with an. aperture member 101A having an aperture opening 101 which allows light to pass therethrough. Further, unused surfaces 102 and 103 are formed to have the minute convex/concave patterns, on which a black ink is to be applied.
Conventionally, in viewing optical systems employed in binoculars, real image camera finders, or the like, prisms such as Porro prisms, roof prisms (dach prisms), penta prisms have been used as an erecting optical system. Recently, optical systems are demanded to be made compact. For this purpose, prisms are designed such that a ratio of a diameter of the flux of light to an area on which the flux of light is incident is as great as possible. In other words, it is preferable that the light transmitting surfaces and light reflecting surfaces are designed to have less marginal areas (areas where the light is not incident).
For example, as shown in FIG. 5, prism 100xe2x80x2 which has less length in the up-and-down direction (i.e., a height), the shape of the aperture opening 101 is formed as shown in FIG. 5. In the prism 100 having a greater height as shown in FIG. 4, the aperture opening 101 has substantially a circular shape, and accordingly, the width xe2x80x9cdxe2x80x9d of the marginal area (i.e., a distance between the edge of the opening 101 and an unused-surface side of the aperture member 101A) is smallest at a position indicated by arrows, and is greater at positions apart from the indicated position. In the prism 100xe2x80x2 having less height as shown in FIG. 5, within a range of xe2x80x9caxe2x80x9d along the right-and-left direction in FIG. 5, the width xe2x80x9cdxe2x80x9d of the marginal area extends.
If such marginal areas extends within a relatively wider range along the unused surface side of the aperture member 101A, it becomes difficult to sufficiently prevent the light that is reflected on the unused surfaces, from emitting through an aperture opening 101 provided on a light emerging surface of the prism. The light which is reflected by the unused surfaces, and passed through the aperture opening is incident on eyes of a user as a ghosting light which prevents the user from viewing image to be observed.
In Japanese Patent Provisional Publication HEI 7-13005, the ghosting light is scattered by minute convex and concave portions formed on a non-unused surface (i.e., a surface where the light is incident, a surface from which the light is emitted, or a surface which reflects the light) at a portion other than a portion where a normal light (i.e., the light which proceeds a normal optical path as designed) is incident, in order to weaken the ghosting light. In the above-described publication, the minute convex and concave portions are formed on a surface which satisfies a total reflection condition (i.e., the normal light incident on the surface is totally reflected by the surface).
In the conventional prism as described above, however, although the ghosting light passed through or reflected by the minute convex and concave portions may be weakened, a part of the ghosting light still remains. Further, if a glass prism is used, process of forming the minute convex and concave portions should be included in its manufacturing process, which increases a manufacturing cost.
It is therefore an object of the invention to provide a prism which prevents generation of ghosting light without increasing manufacturing cost, and to provide a viewing optical system using such a prism.
For the object, according to the present invention, there is provided a prism having a light incident surface, a light emerging surface, and at least one light reflection surface, light incident on the light incident surface being reflected, inside the prism, by the light reflection surface and emerged from the light emerging surface, at least one light reflection surface comprises a reflecting surface which does not satisfy a total reflection condition, the reflecting surface having a reflection area corresponding to a portion on which a normal light is incident. To the light reflection area, light reflecting material is applied. A portion other than the reflection area being formed to be a low-reflection area.
Since a portion of a reflection surface, which does not satisfy the total reflection condition, is formed to be a reflection area, and a portion other than the reflection area is formed to have lower reflectance than the reflection area or to absorb light, the ghosting light is prevented from being emerged from the prism.
Optionally, light absorbing material may be applied to the low-reflection area. An example of the light absorbing material is a black ink.
Further optionally, light reflecting material may be applied to the reflection area. In this case, light absorbing material may applied to the light reflecting surface with covering the light reflecting material.
In this case, the light reflecting material is aluminum or silver.
According to another aspect of the invention, there is provided a viewing optical system, comprising: an objective optical system, which forms an inverted image; an erecting optical system, which converts the inverted image to an erected image; an ocular optical system to be used for viewing the erected image, wherein the erecting optical system includes at least one prism having a plurality of reflection surfaces, one surface, which does not satisfy a total reflection condition, of the plurality of reflection surfaces has a reflection area, to light reflecting material is applied, corresponding to a portion on which a normal light is incident, a portion of the one surface other than the reflection area being formed to be a low-reflection area which has a lower reflectance than the reflection area.
Optionally, the portion on which the normal light is incident is determined in accordance with an optical path of light passing through a field aperture.